Mercury (Hg) emissions have become a health and environmental concern because of their toxicity and ability to bioaccumulate. The U.S. Environmental Protection Agency (EPA) has issued regulations for the control of mercury emissions from waste-to-energy, cement production, and coal-fired power plants. Mercury in flue gas from industrial sources (e.g., power plants) can be captured by sorbents such as activated carbon, which can then be removed by particulate separation devices. The amount of standard sorbents (e.g., activated carbon) needed to serve the market is large. Standard sorbents are not always effective and become more expensive as larger amounts are used.
Inhibition of mercury capture from gas stream by carbon-based sorbents can occur when sulfur (VI) (e.g., SO3, H2SO4) is present in the gas stream, with increasing inhibition at higher concentrations. Low sulfur coals when burned can produce SO3 concentrations in the flue gas from 1-5 ppm, whereas high sulfur coals when burned can produce SO3 concentrations in the flue gas in excess of 30 ppm. Mercury capture above sulfur (VI) concentrations of 3 ppm by mole is limited. Many utilities desire to operate SO3 injection systems at a minimum of about 5-6 ppm to improve ash collectability. However, a sulfur (VI) concentration of about 6 ppm can diminish elemental mercury capture by about 25%-50% or more. With this reduction, it becomes difficult, if not impossible, to economically achieve desired mercury levels. The scientific understanding of why a severe inhibition of mercury sorption exists when sulfur (VI) concentrations increase by such a small amount is limited.